One of the challenges of the cryogenic treatment of flows charged with CO2 for separating the latter by partial condensation is to avoid suddenly freezing the liquid CO2 that is generally found close to the triple point.
Indeed, in order to optimize the separation energy and above all the CO2 recovery yield, it is advisable to cool the mixture from which it is desired to extract the CO2 as cold as possible. The physical limit that appears is that of the solidification temperature of the liquid obtained by partial condensation.
In the prior art, it was known to vaporize the liquid CO2 at the lowest possible pressure in order to provide the cold needed for the partial condensation. Thus, liquid CO2 that is almost pure is vaporized at a pressure as close as possible to the triple point since it is in this way that the coldest temperature is generated. The vaporized CO2 is heated and compressed in order to serve as cycle molecules (in the case of a CO2 liquefier) or in order to be exported as product or for both applications.
This process is efficient since it permits a high CO2 recovery yield at a relatively low energy cost (compared to the alternatives). It also offers the possibility of not introducing other coolant gases onto the site, especially in the context of a CO2 liquefier.
The main drawback is that, in the case of depressurization of zones containing liquid CO2, and mainly of the zone corresponding to the vaporization of the CO2 at low pressure which is closest to the triple point, there is a risk of rapidly expanding the liquid with production of two phases: solid and gaseous. Specifically, the CO2 phase diagram prohibits the liquid phase at a pressure of less than 5.1 bar approximately.
This solid CO2 could block the pipes and especially the channels of a plate exchanger. Moreover, the sublimation or melting of this solid CO2 will be difficult since assuming that a liquid or solid fraction is trapped between two plugs of ice, the change of state could lead to the equipment breaking via overpressure. This risk during heating is accentuated by the fact that CO2 ice (solid CO2) is denser than liquid CO2, thus, during freezing, there is little chance of breaking equipment (unlike what happens with water).